The mucosal membranes are one of the largest organs in the body, and comprise the linings of the gastrointestinal, urogenital, and respiratory tracts. These mucosal membranes, while located in the body, are actually physical barriers between the external environment and the sterile internal body cavity known as the systemic environment. Thus, an important function of the mucosal membranes is to keep invading pathogens out of the sterile body cavity. Indeed, a vast majority of human pathogens, including bacteria, viruses, parasites and fungi, initiate infections at the mucosal surfaces (Ogra et al., Clin Microbiol Rev. 14(2):430-45, 2001).
Mucosal immunity is important because stimulation of the mucosal immune response can result in the production of protective B cells and T cells in both mucosal and systemic environments so that infections are stopped before the pathogens enter into the interior body cavity (see, e.g., McCluskie et al., Microbes Infect. 1(9):685-98; 1999; Rosenthal et al., Semin Immunol. 9(5):303-14, 1997). Despite its important role, very few vaccines specifically target the mucosal immune system.
Vaccinations can be either passive or active. Canonically, active vaccinations involve the exposure of an individual's immune system to one or more foreign molecules that elicit an endogenous immune response resulting in the activation of antigen-specific naive lymphocytes that subsequently leads to antibody-secreting B cells or antigen-specific effector and memory T cells. This approach can result in long-lived protective immunity that can be boosted from time to time by renewed exposure to the same antigenic material. The prospect of longevity of a successful immune response to active vaccination makes this strategy more desirable in most clinical settings than passive vaccination whereby a recipient is injected with preformed antibodies or with antigen-specific effector lymphocytes, which can confer rapid protection, but typically do not establish persistent immunity.